Iowa state senator David Johnson likened Trump’s rise to that of Adolf Hitler.
Photograph: Chris Carlson/AP

The Iowa state senator David Johnson became the first elected official to leave the Republican party over Donald Trump on Tuesday, likening the presumptive nominee’s campaign to the rise of Adolf Hitler.

Johnson announced that he was changing his registration to No Party after Trump levelled accusations of bias at Judge Gonzalo Curiel, an American judge of Mexican heritage who allowed the release of some unflattering documents from a case against Trump University.

“I haven’t supported Mr Trump at any point along the way but what I am calling his racist remarks and judicial jihad is the last straw,” Johnson told the Guardian.

Johnson compared Trump’s run for the Republican nomination to the rise of Hitler and said Trump won “by reducing his campaign to reality TV and large crowds and divisive language and all the trappings of a good show for those who like that kind of approach, and that’s what happened in the 1930s in Germany”.

He added: “I think that’s all I need to say, but certainly the fascists took control of Germany under the same types of strategies.”

Johnson also condemned Trump’s proposed ban on Muslims entering the US. Referencing his own father, who was in the first American unit to liberate a Nazi prison camp, Johnson said: “I was raised without hearing any racial slur, any racial epithet. It’s something that if we’re going to exclude Muslims from traveling to the United States, who’s next? Are we going to come down on Jews? ... He’s not fit to be president.”

Johnson also expressed his concern that there was “definitely an innate bigotry” among a large share of Trump voters. “It really hurts for me to say that, but it’s true,” he said.

The former Republican was still hopeful that the GOP would “dump Trump” at the convention in Cleveland.

“I want to be among those in the party who are willing to live up to Republican principles,” Johnson said.

He also echoed other anti-Trump Republicans in their frustration with the choices presented in the election. “With Mr Trump and Secretary Clinton, is this really the best the country can do?” Johnson asked. “Is this where we are at, and what does that say about where we are as a people and where we are as a republic, not even 250 years from our founding?”

Johnson also condemned his former fellow Republicans such as Paul Ryan, who, he said, were willing to denounce Trump’s comments as racist, but who would still vote for him.

Johnson, who has served 18 years in the state legislature, was first elected to the state senate in 2002 after two terms in the state house.

He first supported Rick Perry and then Carly Fiorina in this year’s Iowa caucuses, and expressed comfort with his decision to leave his party.

Johnson said he didn’t know how his constituents were going to react to his decision, but he said he thought he had a responsibility to them to show “leadership and statesmanship” and “take that first big step”.